1. Field of the Invention
The present invention generally relates to wireless communications, and more specifically, to a system and a method for impulse radio power control.
2. Related Art
Recent advances in communications technology have enabled an emerging, revolutionary ultra wideband technology (UWB) called impulse radio communications systems (hereinafter called impulse radio).
Impulse radio was first fully described in a series of patents, including U.S. Pat. No. 4,641,317 (issued Feb. 3, 1987), U.S. Pat. No. 4,813,057 (issued Mar. 14, 1989), U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990) and U.S. Pat. No. 5,363,108 (issued Nov. 8, 1994) to Larry W. Fullerton. A second generation of impulse radio patent include U.S. Pat. No. 5,677,927 (issued Oct. 14, 1997), U.S. Pat. No. 5,687,169 (issued Nov. 11, 1997) and U.S. Pat. No. 5,832,035 (issued Nov. 3, 1998) to Fullerton et al. These patent documents are incorporated herein by reference.
Uses of impulse radio systems are described in U.S. patent application Ser. No. 09/332,502, entitled, xe2x80x9cSystem and Method for Intrusion Detection Using a Time Domain Radar Array,xe2x80x9d and U.S. patent application Ser. No. 09/322,503, entitled, xe2x80x9cWide Area Time Domain Radar Array,xe2x80x9d both filed the same day as the present application, Jun. 14, 1999, both of which are assigned to the assignee of the present invention, and both of which are incorporated herein by reference.
Basic impulse radio transmitters emit short pulses approaching a Gaussian monocycle with tightly controlled pulse-to-pulse intervals. Impulse radio systems typically use pulse position modulation, which is a form of time modulation where the value of each instantaneous sample of a modulating signal is caused to modulate the position of a pulse in time.
For impulse radio communications, the pulse-to-pulse interval is varied on a pulse-by-pulse basis by two components: an information component and a pseudo-random code component. Unlike direct sequence spread spectrum systems, the pseudo-random code for impulse radio communications is not necessary for energy spreading because the monocycle pulses themselves have an inherently wide bandwidth. Instead, the pseudo-random code of an impulse radio system is used for channelization, energy smoothing in the frequency domain and for interference suppression.
Generally speaking, an impulse radio receiver is a direct conversion receiver with a cross correlator front end. The front end coherently converts an electromagnetic pulse train of monocycle pulses to a baseband signal in a single stage. The data rate of the impulse radio transmission is typically a fraction of the periodic timing signal used as a time base. Because each data bit modulates the time position of many pulses of the periodic timing signal, this yields a modulated, coded timing signal that comprises a train of identically shaped pulses for each single data bit. The impulse radio receiver integrates multiple pulses to recover the transmitted information.
In a multi-user environment, impulse radio depends, in part, on processing gain to achieve rejection of unwanted signals. Because of the extremely high processing gain achievable with impulse radio, much higher dynamic ranges are possible than are commonly achieved with other spread spectrum methods, some of which must use power control in order to have a viable system. Further, if power is kept to a minimum in an impulse radio system, this will allow closer operation in co-site or nearly co-site situations where two impulse radios must operate concurrently, or where an impulse radio and a narrow band radio must operate close by one another and share the same band.
In some multi-user environments where there is a high density of users in a coverage area or where data rates are so high that processing gain is marginal, power control may be used to reduce the multi-user background noise to improve the number of channels available and the aggregate traffic density of the area.
Thus, one area in which further improvement is desired is in power control for impulse radio systems. Briefly stated, power control generally refers to adjusting the transmitter output power to the minimum necessary power to achieve acceptable signal reception at an impulse radio receiver. If the received signal power drops too low, the transmitter power should be increased. Conversely, if the received signal power rises too high, the transmitter power should be decreased. This potentially reduces interference with other services and increases the channelization (and thus, capacity) available to a multi-user impulse radio system.
Power control for impulse radio systems have been proposed. For example, in their paper entitled, xe2x80x9cPerformance of Local Power Control In Peer to Peer Impulse Radio Networks with Bursty Traffic,xe2x80x9d Kolenchery et al. describe the combined use of a variable data rate with power control. Kolenchery et al. propose a system that uses closed loop power control with an open loop adjustment of power associated with each change in data rate to maintain constant signal to noise during the transient event of changing the data rate. However, the system proposed by Kolenchery et al. does not make fill use of the properties of UWB. Further, Kolenchery et al. do not describe a system and method for measuring signal quality and applying such a system and method to power control.
A need therefore exists for an improved system and a method for impulse radio power control.
Briefly stated, the present invention is directed to a system and method for impulse radio power control. A first transceiver transmits an impulse radio signal to a second transceiver. A power control update is calculated according to a performance measurement of the impulse radio signal received at the second transceiver. The transmitter power of either transceiver, depending on the particular embodiment, is adjusted according to the power control update.
An advantage of the current invention is that interference is reduced. This is particularly important where multiple impulse radios are operating in close proximity (e.g., a densely utilized network), and their transmissions interfere with one another. Reducing the transmitter power of each radio to a level that produces satisfactory reception increases the total number of radios that can operate in an area without excess interference.
Another advantage of the current invention is that impulse radios can be more energy efficient. Reducing transmitter power to only the level required to produce satisfactory reception allows a reduction in the total power consumed by the transceiver, and thereby increases its efficiency.
Various performance measurements are employed according to the current invention to calculate a power control update. Bit error rate, signal-to-noise ratio, and received signal strength are three examples of performance measurements that can be used alone or in combination to form a power control update. These performance measurements vary by accuracy and time required to achieve an update. An appropriate performance measurement can be chosen based on the particular environment and application.
In one embodiment, where a pulse train including a quantity Ntrain of pulses is transmitted for each bit of information, the output power of a transceiver is controlled by controlling the quantity Ntrain of pulses according to the power control update. For example, in an embodiment where the quantity Ntrain of pulses includes a quantity Nperiod of periods, and each period includes a quantity Npulses-per-period of pulses, the output power of a transceiver can be controlled by controlling the quantity Nperiod of periods. Alternatively, the output power can be controlled by controlling the quantity Npulses-per-period of pulses.
In one embodiment, where the output power of the first transceiver is controlled, the power control update is determined at the second transceiver and then sent from the second transceiver to the first transceiver. Alternatively, the second transceiver sends at least one performance measurement to the first transceiver, and the first transceiver then determines the power control update based on the performance measurement(s).
Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings.